The actual unit from GV is the same no matter what you put it on. What changes is what's between it and the trans. My unit came off of a Ford RV that was parted out in the wrecking yard. I was able to exchange, not free, for the extension housing that was required for what I was doing. The factory overdrive trans units, 42rh/re,46rh/re, 47rh/re, 48re, a500, 518, 618 you might be referring to cannot have the od unit from one of them retro fitted onto the 904 or 727. Totally different main cases. There are some companies that have ways to retro fit one of the those Mopar od trannies to a b/rb/hemi, unless you are running a small block then you are good to go provided you get one off of a 3.9, 5.2 or 5.9 gas. Also on the GM stuff, you might be thinking of what places like Silver Sport offer. Their unit is based off of a GM trans with a bell housing to match other engines. Is this what you were asking?